LAUSR

Significance Vaccines support controlling and eliminating infectious diseases. As most vaccines protect both vaccinated individuals and the society, vaccination is a prosocial act. Its success relies on a large number of contributing individuals. We study whether vaccination is a social contract where individuals reciprocate and reward others who comply with the contract and punish those who don’t. Four preregistered experiments demonstrate that vaccinated individuals indeed show less generosity toward nonvaccinated individuals who violate the social contract. This effect is independent of whether the individuals are members of the same or different social groups. Thus, individuals’ behavior follows the rules of a social contract, which provides a valuable basis for future interventions aiming at increasing vaccine uptake by emphasizing this social contract. Most vaccines protect both the vaccinated individual and the society by reducing the transmission of infectious diseases. In order to eliminate infectious diseases, individuals need to consider social welfare beyond mere self-interest—regardless of ethnic, religious, or national group borders. It has therefore been proposed that vaccination poses a social contract in which individuals are morally obliged to get vaccinated. However, little is known about whether individuals indeed act upon this social contract. If so, vaccinated individuals should reciprocate by being more generous to a vaccinated other. On the contrary, if the other doesn’t vaccinate and violates the social contract, generosity should decline. Three preregistered experiments investigated how a person’s own vaccination behavior, others’ vaccination behavior, and others’ group membership influenced a person’s generosity toward respective others. The experiments consistently showed that especially compliant (i.e., vaccinated) individuals showed less generosity toward nonvaccinated individuals. This effect was independent of the others’ group membership, suggesting an unconditional moral principle. An internal metaanalysis (n = 1,032) confirmed the overall social contract effect. In a fourth experiment (n = 1,212), this pattern was especially pronounced among vaccinated individuals who perceived vaccination as a moral obligation. It is concluded that vaccination is a social contract in which cooperation is the morally right choice. Individuals act upon the social contract, and more so the stronger they perceive it as a moral obligation. Emphasizing the social contract could be a promising intervention to increase vaccine uptake, prevent free riding, and, eventually, support the elimination of infectious diseases.